Oh this reminds me of how we used to drive to Florida. The prime spot was that shelf like area underneath the rear window ( in old cars , before we got the old station wagon. ) It was big enough for a little kid to lie down on and the sun felt good. Who cares that my dad couldn’t see out the back window, or if he stopped fast, said child would go flying….Can't believe you, of all people, have fallen for the Helicopter Parenting Industrial Complex.
You don't need a car seat. We did just fine bouncing around on the old bench seats or riding in dad's lap. Grow a set, dude. Cray would be disgusted with you.